The following account of the prior art relates to one of the areas of application of the present application, hearing aids.
The basic idea of frequency compression or frequency transposition in general is to make frequencies, that are inaudible for a person (having a specific hearing impairment) with conventional amplification, audible by moving them. The fact that it is not possible—with conventional hearing aids—to compensate a hearing impairment at some frequencies can have several reasons. The two most likely reasons are 1) that the amplification cannot be made high enough due to feedback oscillation issues, or 2) that the patient has “dead regions”, where hearing ability is severely degraded or non-existent. Dead regions theoretically would indicate regions of the basilar membrane where the sensory cells (the inner hair cells) do not function. Very strong amplification would then not help that location of the basilar membrane. Frequency lowering or transposition could in such cases be a solution, where information at an inaudible frequency is moved to an audible range.
Nonlinear frequency compression (NFC) has so far shown the best results of the different frequency lowering techniques (see [Simpson; 2009] for an overview of different signal processing approaches). NFC has been shown to improve speech intelligibility for hearing impaired users in certain circumstances. In NFC, the frequency axis is divided into a linear part and a compressed part (cf. e.g. FIG. 1a showing a non-compressed (linear, fin=fout) part and a compressed (fin>fout) part at frequencies, respectively, below and above a predetermined cut-off frequency, fcut).
WO 2005/015952 (Vast Audio) describes a system that aims at improving the spatial hearing abilities of hearing-impaired subjects. The proposed system discards every nth frequency analysis band and pushes the remaining ones together, thus applying frequency compression. As a result, spatially salient high-frequency cues are assumed to be reproduced at lower frequencies.
EP 1 686 566 A2 (Phonak) deals with a signal processing device comprising means for transposing at least part of an input signal's spectral representation to a transposed output frequency, the frequency transposition means being configured to process the portion of the input signal spectral representation such that a phase relationship that existed in the input signal's spectral representation is substantially maintained in the transposed portion of the spectral representation.
EP 2 091 266 A1 (Oticon) deals with the transformation of temporal fine structure-based information into temporal envelope-based information in that a low frequency source band is transposed to a high frequency target band in such a way that the (low-frequency) temporal fine structure cues are moved to a higher frequency range. Thereby the ability of hearing-aid users to access temporal fine structure-based cues can be improved.